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General control line discussion => Open Forum => Topic started by: Kafin Noe’man on January 03, 2025, 11:33:00 PM
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Hi all,
I was wondering what are the things to consider when you convert a full built-up fuselage design planes into a profile version of it?
I somehow prefer profile fuselage due its practicality aspect when you need to make adjustment later on, for example: tank height, engine offset, fuel lines, etc.
Best,
Kafin
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I am sure there will be a bunch of others with more advice but I have converted most of my designs to profile at one time or another. Aside from the side mount of the motor which will raise the vertical CG just a bit the two major things that are different are stab mount and the bellcrank position. The stab needs to be mounted as best you can to avoid movement. In a full body you have a wide mount and a body that does not flex or twist very much. A profile will flex and twist a lot. This will throw the stab out of alignment in flight. Making the aft fuselage as light and stiff as possible is very important. One thing I do to compensate for the flex is measure it before gluing the stab in. If, and only if, it flexes a lot, I glue the stab in with the inboard tip 1/32" closer to the wing TE which will make it parallel in flight.
The controls will be on the outside. You will need to position the bellcrank to keep the pushrod as straight as possible. A profile flap horn will not be centered so you need to have the controls exit on the inboard wing so that whatever twist there might be in the horn produces outboard roll in flight.
I am glad to see you starting to have a go at design. To me that is the most rewarding part of the hobby. Have fun - Ken
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Make the back half of the fuselage a truss design with 1/16 balsa covering on a 45 degree bias or 1/64 ply covering. Find a drawing of the MO' BEST that will explain it. Pat Johnston's designs are similar. By far the easiest is to get some .041 x .147 carbon strip and glue a piece across the aft portion of the fuselage, canopy to tail wheel on one side and above the stab to below the wing on the other. Forms an "X" like the bias balsa. I found some of the carbon strip on a wrecked combat plane. The builder uses it as a spar cap strip.
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Something to consider if you want the airplane to perform well or at least close to how the full body version does is the vertical CG. Most common full fuselage designs have the engine mounted inverted with the wing mounted below the thrust line perhaps 3/4" to 1" to compensate for the mass of the engine, muffler and fuel load hanging below the thrust line. When you convert this into a profile ( and maintain the original thrust line) now all the hardware is well above the wing centerline causing an outward rolling force in flight. To overcome that you would raise the wing much closer to centerline and lengthen the landing gear for a wing mounted gear. Honestly by the time you've done all this you have an entirely different design than you started with- maybe better to just pick a good known profile and build that. Or if you already have the wing and empenage components then just draw up your own scratch fuselage to use them keeping in mind you'll need to keep the thrust and wing centerlines within about 1/2" or less of one another. and ALL these lines including the stab perfectly parallel to each other from the side view.
Dave
To add another thought as well; the engines we use most always run better-more evenly- when mounted in the upright or inverted position than side mounted on a profile. About the only exception to this I've seen is the Discovery Retro motors as used in the IC powered Shark. Profiles also have a much greater tendency to manifest vibration which is detrimental to good engine runs. If quality of engine runs and performance is important to you you will be better off to adapt to full fuselage airplanes anyway.
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A really good way for the engine is to do like the late Al Rabe did with the Mu stunt profile. using the truss construction for the rear of the fuselage. This will get you used to starting engines for the full fuse planes. S?P
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Ted Fancher's "Imitation" is a profile that allows mounting the engine with the cylinder in any direction, with one of those glass reinforced R/C mounts. But one of the best I've seen was Jim Rhodes' with a conventional profile nose and side-mounted Aero Tiger .36. Big airplane and thick wing for a .36, but it was very light and appeared to fly great. y1 Steve
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Yacan putathing init , like that , for the TANK . If the Outlet is level / inboard of the N V A , It'll run richer lower & leaner higher . Set soit dosnt ' Dry Off ' in the wingover / Overhead Eights . To lean there leads to tears .
(https://stunthanger.com/smf/open-forum/whos-silly-idea-was-that-!/?action=dlattach;attach=241744)
AND putathingon the iboard side , back to over the spar . Putting the AFT taper face beforehand , means gougeing & stress raisers arnt init . This is cause it's a ENGINE BED . So as it absorbes the shake rattle & roll .
(http://www.controlline.org.uk/userupload/613/0005.jpg)
One can do a inverted nose Fwd. & profile aft . ifin yr engine DOES wanna go beta up or down right . SOME put the Engine Inboard - so the dampness is away from the plug .
https://stunthanger.com/smf/open-forum/frank-williams-profile-bearcat/msg643338/#msg643338
(https://library-modelaviation.s3.us-east-2.amazonaws.com/styles/full_page/s3/ma/ma198312/ma198312_076.jpg)
One selects the stiffest sheet , if its a 1/2 sheet profile . ONCE that meant the slowest growed tree , which was light , and stiff as a plank .